Dual bit memory cells are known in the art. One such memory cell is the NROM (nitride read only memory) cell 10, shown in FIG. 1 to which reference is now made, which stores two bits 12 and 14 in a nitride based layer 16 sandwiched between a conductive layer 18 and a channel 20. NROM cells are described in many patents, for example in U.S. Pat. No. 6,649,972, assigned to the common assignees of the present invention, whose disclosure is incorporated herein.
Bits 12 and 14 are individually accessible, and thus, may be programmed (conventionally noted as a ‘0’), erased (conventionally noted as a ‘1’) or read separately. Reading a bit (12 or 14) involves determining if a threshold voltage Vt, as seen when reading the particular bit, is above (programmed) or below (erased) a read reference voltage level RD.
FIG. 2, to which reference is now made, illustrates the distribution of programmed and erased states of a memory chip (which typically has a large multiplicity of NROM cells formed into a memory array) as a function of threshold voltage Vt. An erased bit is one whose threshold voltage has been reduced below an erase threshold voltage EV. Thus, an erase distribution 30 has typically its rightmost point in the vicinity of (and preferably at or below) the erase threshold voltage EV. Similarly, a programmed bit is one whose threshold voltage has been increased above a program threshold voltage PV. Thus, a programmed distribution 32 has typically its leftmost point in the vicinity of (and preferably at or above) the program threshold voltage PV.
The difference between the two threshold voltages PV and EV is a window W0 of operation. Read reference voltage level RD is typically placed within window W0 and can be generated, as an example, from a read reference cell. The read reference cell is usually, but not necessarily, in a non-native state, as described in U.S. Pat. No. 6,490,204, assigned to the common assignee of the present invention, whose disclosure is incorporated herein by reference. In such case, the threshold voltage of read reference cell may be at the RD level in FIG. 2.
The signal from the bit being read is then compared with a comparison circuit (e.g. a differential sense amplifier) to the signal generated by the read reference level, and the result should determine if the array cell is in a programmed or erased state. Alternatively, instead of using a reference cell, the read reference signal can be an independently generated voltage or a current signal. Other methods to generate a read reference signal are known in the art.
Since the sensing scheme circuitry may not be perfect, and its characteristics may vary at different operating and environmental conditions, margins M0 and M1 are typically required to correctly read a ‘0’ and a ‘1’, respectively. As long as the programmed and erased distributions are beyond these margins, reliable reads may be achieved.
It will be appreciated that for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity. Further, where considered appropriate, reference numerals may be repeated among the figures to indicate corresponding or analogous elements.